This application claims benefit of priority under 35 USC xc2xa7119 to Japanese patent application No. 2001-048432, filed on Feb. 23, 2001, entire contents of which are incorporated by reference herein.
The present invention relates to a semiconductor light emitting device for emitting mixed-color light and more particularly to a semiconductor light emitting device for a white light source.
A semiconductor light emitting diode (hereinafter referred to as LED) for emitting visible lights has characteristics of small size, low power consumption, and high reliability. Therefore, with high brightness of emitted lights being realized, it is used as an outdoors indicator and a communication light source and thus its use is rapidly spread. Such high-brightness LED materials as AlGaAs, GaAlP, GaP, and InGaAlP are in practical use at present, and are supplied at a low cost as element materials respectively for emitting red, orange, yellow, and green colors respectively. In addition, recently, blue and green LEDs using a GaN series material are put into practical use and thus, an LED for emitting the three primary colors of RGB (red, green, blue) at high brightness is realized. Therefore, by combination of a plurality of LEDs emitting RGB lights respectively, full-color indication is made possible.
On the other hand, another advantage of a combination of LEDs emitting RGB lights respectively is that white light can be emitted. It is expected as an illumination source. Advantages produced by replacement by the LEDs of a light electric lamp, a back light source for a liquid crystal indicator, or an automobile indicator are great from the viewpoint of reliability and cost.
Recently, a white light source has been realized by a combination of elements emitting a blue and a yellow light respectively. Specifically, a combination of a GaN series LED emitting a blue light and a yellow light phosphor, or a combination of a ZnSe series LED emitting a blue light and a yellow luminescence substrate is used. Both of the combinations include a blue light emitting LED and are characterized in that they excite and emit yellow rays.
These LEDs for a white light source are characterized in that they have a simple structure and can be miniaturized, and that they have a wide yellow luminescence spectrum band, so that their color rendering is comparatively high although light emitted by them is white light with two colors mixed.
Further, an LED is also proposed for a white light source that a plurality of luminous layers for emitting RGB respectively are formed with an InGaAlN material in an active layer of a single light emission element. This type of LED for a white light source can be more miniaturize than those described above.
However, such LEDs for a white light source as described above, has a problem that yellow rays excited and emitted by a blue luminous LED are unstable.
Further, with respect to an LED for a white light source that a plurality of luminous layers for emitting RGB respectively are formed in an active layer of a single light emission element, it is difficult to bring it into a practical use because sufficient luminous brightness of R cannot be obtained.
Therefore, an object of the present invention is to provide an LED for a white light source, which is small and has sufficient brightness and superior color rendering.
A semiconductor light-emitting device according to the present invention has a first semiconductor light emission element, e. g. a LED, for emitting color light in the first wavelength range and a second semiconductor light emission element, e. g. a LED, arranged in the neighborhood of the first semiconductor light emission element for emitting color light in the second wavelength range. The first semiconductor light emission element is made of an InGaAlP series material, and includes an active layer having a plurality of composite luminous layers for emitting color light with a different wavelength.
Luminescence spectra from the plurality of the composite luminous layers are partially overlapped with each other and each of the plurality of the composite luminous layers includes a plurality of luminous layers for emitting a color light with substantially a same wavelength.
Further, the semiconductor light emitting device according to the present invention is characterized in that it has a first semiconductor light emission element, e. g. a LED, for emitting color light in the first wave length range and a second semiconductor light emission element, e. g. a LED, arranged in the neighborhood of the first semiconductor light emission element for emitting color light in the second wave length range, and the first semiconductor light emission element is composed of an InGaAlP series material, and the light emission element composed of the InGaAlP series material has a plurality of luminous layers for emitting color light with a different wave length, and the luminescence spectra from the plurality of luminous layers are partially overlapped with each other.
In more concretely, the semiconductor light-emitting device according to the present invention is composed of two semiconductor light emission elements for emitting different kinds of color light, for example, blue and yellow light, which are complementary colors and are arranged adjacently on a same frame electrode. One of the elements for emitting the yellow light is made of an InGaAlP series material, which is arranged to have a wide-band luminescence spectrum.
Namely, the InGaAlP material has a band structure in which direct transition from red to green takes place, so that it is already widely put into practical use as a high-brightness LED material within those range of wavelength. The inventor noticed that a white light source of high brightness can be put into practical use by a combination of an InGaAlP series yellow LED and a blue LED made of GaN series material. However, on the other hand, it is found that the luminescence spectrum of the GaN series blue LED is comparatively wide, while the luminescence spectrum of the InGaAlP series LED of high brightness is narrow because the luminescence is based on the band structure of the direct transition and the color rendering of the mixed color is not sufficiently high.
FIG. 1 shows a luminescence spectrum of a white light emission element relating to the conventional embodiment. In this case, the luminescence having a peak at a wavelength of about 490 nm is from the GaN series LED and the luminescence having a peak at a wavelength of about 590 nm is luminescence from the InGaAlP series LED. The LEDs are mounted on the same frame and operated at an operation current of 20 mA. As shown in the drawing, the luminescence from the InGaAlP series LED has a narrow full width at half maximum and the color rendering is expected to be low, though white luminescence can be obtained by the mixed color.
According to the present invention, a plurality of luminous layers for emitting color light with a different wave length are formed in a light emission element made of an InGaAlP series material, thereby widening the luminescence spectrum. The color rendering of mixed-color light or white light obtained from two semiconductor light emission elements is improved.